Determining optimum conditions for accurate and precise dosimetry continues to be a major concern in radiation therapy. Indeed in all methods of radiation treatment it is imperative to choose operating conditions which provide effective radiation delivery to an afflicted tissue volume, while minimizing the impact on healthy outlying tissue. Conventional radiation therapy utilizes electron beams, x-rays or gamma rays as ionizing radiation applied to malignant tissue. To this end, accurate, repeatable delivery systems and calibration procedures have been developed, whereupon once an appropriate treatment plan is established, the plan may in principle be administered at any equivalent delivery system without modification or recalibration. In this respect, conventional radiation therapy may be regarded much like any other administered medication where techniques and standards have established a high degree of accuracy and precision. However, unlike many other forms of medication, radiation can be administered only at the time of its creation.
Photons such as X-rays or .gamma.-rays generally deposit a radiation dose in an exponentially attenuated manner with tissue penetration depth, thus disturbing much tissue before reaching a desired target volume. High energy proton beams on the other hand present several distinct advantages residing principally in their physical characteristics. Protons carry more momenta for a given beam energy, creating a highly forward directed beam upon tissue traversal. Due to their mass and charge, the rate of energy loss from a proton beam rises as the beam loses energy, reaching a maximum near the stopping point or so-called Bragg peak. Thus in comparison to other forms of radiation, proton beams offer the possibility of delivering more effective therapeutic doses directed principally within well-defined target volumes. These conditions also mean that proton delivery systems should be at least as accurate and precise as their conventional counterparts.
In order to exploit these advantages, technology must be developed to facilitate clinically useful proton beams; specifically, accurate and precise dose delivery over time, and component exchange. In most proton therapy systems around the world, the proton accelerators were originally built for physics research and later adapted for part-time clinical research and therapy. One of the basic components of a proton therapy facility is a beam delivery system capable of delivering precise dose distributions to the target volume within a patient. To this end, a beam delivery system may comprise, for example, a beam spreading device to produce a large, uniform field, a range modulator to disperse the Bragg peak and various beam detectors to measure fluence, centering and dose distribution. A more detailed description of beam delivery system components and their function is provided in an article by Coutrakon et al., "A Prototype Beam Delivery System for the Proton Medical Accelerator at Loma Linda," Medical Physics, vol. 18, no. 6, 1093-1099 (1991).
To understand and predict dose delivery, a suitable dynamic model of proton transport and energy deposition could be developed, specific to the type of delivery system and capable of establishing dosimetric capability in a deterministic manner. Unfortunately however, previous attempts to model dose-distribution have not succeeded in reaching the level of accuracy established by accepted medical practice. As is commonly done, a calibration is performed for every treatment plan to be administered. To ensure accurate dosimetry for a prescribed treatment plan, delivery system radiation monitors are precalibrated with an absolute standard under the conditions of treatment. The method of individual treatment plan calibration remains the standard practice.
The imminent advantages of proton beam therapy can be realized only with the development of dedicated, clinically based facilities. One such therapy facility, located at the Loma Linda University Medical Center, was purposely built to provide therapeutic proton beams to a multiplicity of treatment rooms. An overview of the facility and its development is provided in "The Proton Treatment Center at Loma Linda University Medical Center: Rationale for and Description of its Development," J. M. Slater et al., Intl. J. Radiation Oncology, vol. 22, no. 2, 1992, pp. 383-389. Having one proton synchrotron source, the proton beam itself can be delivered to only one treatment station at time. Treatment calibration, however, is a time consuming operation and represents the majority of operating time of an expensive proton beam facility. As such, a lengthy calibration period for each treatment plan severely limits the efficiency and potential cost reduction offered by a clinically-based, multiple treatment center facility. Hence, while the method of individual treatment plan calibration may be acceptable for an experimental or developmental proton beam facility, it is clearly insufficient for the operation of a clinically based proton beam therapy facility.